1. Field of the Invention
The present invention relates to control devices configured to provide user control signals, in particular control devices providing two dimensional control information such as a joystick.
2. Description of the Prior Art
Control devices which provide user control signals based on the relative orientation of a magnetic flux sensor and a magnet are known. For example, a joystick may be constructed based on a familiar ball and socket arrangement, wherein a permanent magnet attached to the ball may be orientated with respect to a magnetic flux sensor mounted in the socket by means of a joystick handle protruding from the ball.
One such joystick arrangement is disclosed in US Published Patent Application 2006/0028184 (and is schematically illustrated in FIG. 1). This arrangement comprises a shaft 11 forming the joystick handle terminating in a ball 12 which encloses a permanent magnet 13. The ball 12 is mounted such that it may be rotated around two axes of rotation, namely when the joystick shaft 11 is pushed forwards-and-backwards or left-and-right. Surrounding the ball is a pole-piece frame arrangement formed of a material with a high magnetic permeability composed of four collector plates 18A, 18B, 18C and 18D equally spaced around the ball with four pole-piece arms 19A, 19B, 19C and 19D. The pole-piece arms connect the four collector plates to the corners of the pole-piece frame arrangement, where four pairs of plates 20A, 20B, 20C and 20D are arranged. Gaps 21A, 21B, 21C and 21D are formed between the four pairs of plates and in two of the gaps on one side of the pole-piece frame arrangement are a pair of identical Hall-effect sensors 22 aligned side-by-side. As the joystick shaft is moved through its range of movement, this changes the magnetic flux experienced in each of the gaps 21A and 21D (and hence by each pair of Hall-effect sensors 22). For example, if the joystick shaft 11 is angled towards gap 21A this will cause collector plates 18A and 18B to experience a biased magnetic polarity with respect to that experienced by collector plates 18C and 18D. Because of this, a magnetic flux will pass through the gaps 21B and 21D. Conversely, since gaps 21A and 21C experience the same magnetic potential no flux will pass across these gaps. The pair of sensors located within gap 21D will experience the flux change and thus generate an electrical signal due to the Hall-effect, which can be translated into a desired control signal by suitable electronics. The pairs of Hall-effect sensors 22 in gaps 21A and 21D provide a level of redundancy for the joystick device. However, the use of four Hall-effect sensors has an associated manufacturing cost. Conversely, one advantage of this joystick arrangement is that it is relatively insensitive to small offsets in the position of ball 12 and magnet 13 relative to the pole-piece frame arrangement, since it is differences in the magnetic polarity between pairs of collector plates which causes the magnetic flux to pass across the measuring gaps. This is advantageous in implementations of the joystick arrangement which are required to be robust, such as the controller of a wheelchair which is in daily use and may often be knocked, leant on and so on.
FIGS. 2A and 2B schematically illustrate two alternative joystick arrangements which make use of the measurement of magnetic flux variations using two-axis Hall-effect sensors. FIG. 2A schematically illustrates a gimbal based arrangement such as that disclosed in the “MLX90333 Triaxis 3D-Joystick Position Sensor” data sheet by Melexis, March 2009 and in the “Angular Position Sensing with Two-Axis Hall IC 2SA-10” note by Sentron (Melexis), February 2004. In this arrangement the joystick shaft 30 has a permanent magnet 32 mounted on its distal end which can be oriented with respect to a two-axis Hall-effect sensor 34 by means of a gimbal arrangement 36. The two-axis Hall sensor 34 is composed of a CMOS chip having a central ferro-magnetic disc and four Hall elements equally spaced around the disc's edge. The ferro-magnetic disc acts as a flux concentrator which converts magnetic field components parallel to the device surface into a component perpendicular to the device surface which can be measured by the Hall elements embedded in the surface of the CMOS chip. The four Hall elements thus provide two dimensional orientation information for the joystick based on a single sensor chip. This is advantageous in terms of manufacturing costs. However, a gimbal arrangement such as that illustrated in FIG. 2A must be carefully constructed such that the magnet 32 only moves within a limited range of movement—close enough to the sensor 34 that the sensor experiences a relatively uniform magnetic field, but at a sufficient distance from the sensor to allow an acceptable range of movement for the joystick without the magnet colliding with the sensor. Furthermore, such gimbal arrangements may have their spatial configuration easily altered by mechanical loads on the joystick, either static, such as a user leaning on the joystick or dynamic such as an impact. Such mechanical loading can easily upset the careful positioning required between the magnet 32 and the sensor 34.
FIG. 2B schematically illustrates another known joystick arrangement using a single Hall sensor integrated circuit 40, wherein the magnet 42 is mounted on the surface of a ball-shaped carrier 44 which is mounted in a socket. The joystick 48 extends from the ball-shaped carrier such that movement of the joystick 48 will cause rotation of the ball-shaped carrier 44 in the socket 46, changing the relative position of the magnet 42 and the Hall sensor 40. Such a joystick arrangement is disclosed in the technical note “AN5011-10 Low Power Hall IC” by Austria Microsystems, July 2008. A sliding plate 50 is urged against the socket 46 by means of a spring 52, causing the joystick 48 to tend to return to its upright position. Whilst this arrangement, like the arrangement in FIG. 2A, can make use of a single two-axis Hall sensor, correct operation of this joystick arrangement is dependent on the ball-shaped carrier 44 engaging the socket 46 in a close fit. Wear between the surfaces of the ball-shaped carrier 44 and the socket 46 will cause the ball-shaped carrier 44 to shift with respect to its original position in the socket 46 as the surfaces abrade due to the friction enhanced by the action of spring 52. Given that typical use of the joystick will result in asymmetric wear at the ball/socket interface, the ball might tend to move upwards and sideways over time. This would cause the relative positioning of magnet 42 and Hall sensor 40, and hence the output of the Hall sensor, to change with time. Furthermore, the arrangement in FIG. 2B is also sensitive to the static and dynamic mechanical loads discussed with reference to FIG. 2A.
Accordingly, it would be desirable to provide a control device which combined the lower manufacturing costs associated with the use of a single integrated circuit Hall sensor but which is more resilient to the mechanical stresses and strains imposed by rough handling without the interaction between magnet and Hall sensor being adversely affected.